The extension of Exim to include content scanning at ACL time, formerly known
as “exiscan”, was originally implemented as a patch by Tom Kistner. The code
was integrated into the main source for Exim release 4.50, and Tom continues to
maintain it. Most of the wording of this chapter is taken from Tom’s
specification.

It is also possible to scan the content of messages at other times. The
local_scan() function (see chapter 44) allows for content
scanning after all the ACLs have run. A transport filter can be used to scan
messages at delivery time (see the transport_filter option, described in
chapter 24).

If you want to include the ACL-time content-scanning features when you compile
Exim, you need to arrange for WITH_CONTENT_SCAN to be defined in your
Local/Makefile. When you do that, the Exim binary is built with:

Two additional ACLs (acl_smtp_mime and acl_not_smtp_mime) that are run
for all MIME parts for SMTP and non-SMTP messages, respectively.

Additional ACL conditions and modifiers: decode, malware,
mime_regex, regex, and spam. These can be used in the ACL that is
run at the end of message reception (the acl_smtp_data ACL).

An additional control feature (“no_mbox_unspool”) that saves spooled copies
of messages, or parts of messages, for debugging purposes.

Additional expansion variables that are set in the new ACL and by the new
conditions.

Two new main configuration options: av_scanner and spamd_address.

There is another content-scanning configuration option for Local/Makefile,
called WITH_OLD_DEMIME. If this is set, the old, deprecated demime ACL
condition is compiled, in addition to all the other content-scanning features.

Content-scanning is continually evolving, and new features are still being
added. While such features are still unstable and liable to incompatible
changes, they are made available in Exim by setting options whose names begin
EXPERIMENTAL_ in Local/Makefile. Such features are not documented in
this manual. You can find out about them by reading the file called
doc/experimental.txt.

All the content-scanning facilities work on a MBOX copy of the message that is
temporarily created in a file called:

<spool_directory>/scan/<message_id>/<message_id>.eml

The .eml extension is a friendly hint to virus scanners that they can
expect an MBOX-like structure inside that file. The file is created when the
first content scanning facility is called. Subsequent calls to content
scanning conditions open the same file again. The directory is recursively
removed when the acl_smtp_data ACL has finished running, unless

control = no_mbox_unspool

has been encountered. When the MIME ACL decodes files, they are put into the
same directory by default.

1. Scanning for viruses

The malware ACL condition lets you connect virus scanner software to Exim.
It supports a “generic” interface to scanners called via the shell, and
specialized interfaces for “daemon” type virus scanners, which are resident
in memory and thus are much faster.

You can set the av_scanner option in first part of the Exim configuration
file to specify which scanner to use, together with any additional options that
are needed. The basic syntax is as follows:

av_scanner = <scanner-type>:<option1>:<option2>:[...]

If you do not set av_scanner, it defaults to

av_scanner = sophie:/var/run/sophie

If the value of av_scanner starts with a dollar character, it is expanded
before use.
The usual list-parsing of the content (see 6.19) applies.
The following scanner types are supported in this release:

aveserver

This is the scanner daemon of Kaspersky Version 5. You can get a trial version
at http://www.kaspersky.com. This scanner type takes one option,
which is the path to the daemon’s UNIX socket. The default is shown in this
example:

av_scanner = aveserver:/var/run/aveserver

clamd

This daemon-type scanner is GPL and free. You can get it at
http://www.clamav.net/. Some older versions of clamd do not seem to
unpack MIME containers, so it used to be recommended to unpack MIME attachments
in the MIME ACL. This no longer believed to be necessary. One option is
required: either the path and name of a UNIX socket file, or a hostname or IP
number, and a port, separated by space, as in the second of these examples:

If the value of av_scanner points to a UNIX socket file or contains the local
keyword, then the ClamAV interface will pass a filename containing the data
to be scanned, which will should normally result in less I/O happening and be
more efficient. Normally in the TCP case, the data is streamed to ClamAV as
Exim does not assume that there is a common filesystem with the remote host.
There is an option WITH_OLD_CLAMAV_STREAM in src/EDITME available, should
you be running a version of ClamAV prior to 0.95.

The final example shows that multiple TCP targets can be specified. Exim will
randomly use one for each incoming email (i.e. it load balances them). Note
that only TCP targets may be used if specifying a list of scanners; a UNIX
socket cannot be mixed in with TCP targets. If one of the servers becomes
unavailable, Exim will try the remaining one(s) until it finds one that works.
When a clamd server becomes unreachable, Exim will log a message. Exim does
not keep track of scanner state between multiple messages, and the scanner
selection is random, so the message will get logged in the mainlog for each
email that the down scanner gets chosen first (message wrapped to be readable):

If the option is unset, the default is /tmp/clamd. Thanks to David Saez for
contributing the code for this scanner.

cmdline

This is the keyword for the generic command line scanner interface. It can be
used to attach virus scanners that are invoked from the shell. This scanner
type takes 3 mandatory options:

The full path and name of the scanner binary, with all command line options,
and a placeholder (%s) for the directory to scan.

A regular expression to match against the STDOUT and STDERR output of the
virus scanner. If the expression matches, a virus was found. You must make
absolutely sure that this expression matches on “virus found”. This is called
the “trigger” expression.

Another regular expression, containing exactly one pair of parentheses, to
match the name of the virus found in the scanners output. This is called the
“name” expression.

For example, Sophos Sweep reports a virus on a line like this:

Virus 'W32/Magistr-B' found in file ./those.bat

For the trigger expression, we can match the phrase “found in file”. For the
name expression, we want to extract the W32/Magistr-B string, so we can match
for the single quotes left and right of it. Altogether, this makes the
configuration setting:

The DrWeb daemon scanner (http://www.sald.com/) interface
takes one option,
either a full path to a UNIX socket,
or host and port specifiers separated by white space.
The host may be a name or an IP address; the port is either a
single number or a pair of numbers with a dash between.
For example:

If you omit the argument, the default path /usr/local/drweb/run/drwebd.sock
is used. Thanks to Alex Miller for contributing the code for this scanner.

f-protd

The f-protd scanner is accessed via HTTP over TCP.
One argument is taken, being a space-separated hostname and port number
(or port-range).
For example:

av_scanner = f-protd:localhost 10200-10204

If you omit the argument, the default values show above are used.

fsecure

The F-Secure daemon scanner (http://www.f-secure.com) takes one
argument which is the path to a UNIX socket. For example:

av_scanner = fsecure:/path/to/.fsav

If no argument is given, the default is /var/run/.fsav. Thanks to Johan
Thelmen for contributing the code for this scanner.

kavdaemon

This is the scanner daemon of Kaspersky Version 4. This version of the
Kaspersky scanner is outdated. Please upgrade (see aveserver above). This
scanner type takes one option, which is the path to the daemon’s UNIX socket.
For example:

av_scanner = kavdaemon:/opt/AVP/AvpCtl

The default path is /var/run/AvpCtl.

mksd

This is a daemon type scanner that is aimed mainly at Polish users, though some
parts of documentation are now available in English. You can get it at
http://linux.mks.com.pl/. The only option for this scanner type is
the maximum number of processes used simultaneously to scan the attachments,
provided that the demime facility is employed and also provided that mksd has
been run with at least the same number of child processes. For example:

av_scanner = mksd:2

You can safely omit this option (the default value is 1).

sock

This is a general-purpose way of talking to simple scanner daemons
running on the local machine.
There are four options:
an address (which may be an IP addres and port, or the path of a Unix socket),
a commandline to send (may include a single %s which will be replaced with
the path to the mail file to be scanned),
an RE to trigger on from the returned data,
an RE to extract malware_name from the returned data.
For example:

av_scanner = sock:127.0.0.1 6001:%s:(SPAM|VIRUS):(.*)\$

Default for the socket specifier is /tmp/malware.sock.
Default for the commandline is %s\n.
Both regular-expressions are required.

sophie

Sophie is a daemon that uses Sophos’ libsavi library to scan for viruses.
You can get Sophie at http://www.clanfield.info/sophie/. The only option
for this scanner type is the path to the UNIX socket that Sophie uses for
client communication. For example:

av_scanner = sophie:/tmp/sophie

The default path is /var/run/sophie, so if you are using this, you can omit
the option.

When av_scanner is correctly set, you can use the malware condition in
the DATA ACL. Note: You cannot use the malware condition in the MIME
ACL.

The av_scanner option is expanded each time malware is called. This
makes it possible to use different scanners. See further below for an example.
The malware condition caches its results, so when you use it multiple times
for the same message, the actual scanning process is only carried out once.
However, using expandable items in av_scanner disables this caching, in
which case each use of the malware condition causes a new scan of the
message.

The malware condition takes a right-hand argument that is expanded before
use. It can then be one of

“true”, “*”, or “1”, in which case the message is scanned for viruses.
The condition succeeds if a virus was found, and fail otherwise. This is the
recommended usage.

“false” or “0” or an empty string, in which case no scanning is done and
the condition fails immediately.

A regular expression, in which case the message is scanned for viruses. The
condition succeeds if a virus is found and its name matches the regular
expression. This allows you to take special actions on certain types of virus.

You can append /defer_ok to the malware condition to accept messages
even if there is a problem with the virus scanner. Otherwise, such a problem
causes the ACL to defer.

When a virus is found, the condition sets up an expansion variable called
$malware_name that contains the name of the virus. You can use it in a
message modifier that specifies the error returned to the sender, and/or in
logging data.

If your virus scanner cannot unpack MIME and TNEF containers itself, you should
use the demime condition (see section 43.6) before the
malware condition.

Beware the interaction of Exim’s message_size_limit with any size limits
imposed by your anti-virus scanner.

2. Scanning with SpamAssassin

The spam ACL condition calls SpamAssassin’s spamd daemon to get a spam
score and a report for the message. You can get SpamAssassin at
http://www.spamassassin.org, or, if you have a working Perl
installation, you can use CPAN by running:

perl -MCPAN -e 'install Mail::SpamAssassin'

SpamAssassin has its own set of configuration files. Please review its
documentation to see how you can tweak it. The default installation should work
nicely, however.

After having installed and configured SpamAssassin, start the spamd daemon.
By default, it listens on 127.0.0.1, TCP port 783. If you use another host or
port for spamd, you must set the spamd_address option in the global
part of the Exim configuration as follows (example):

spamd_address = 192.168.99.45 387

You do not need to set this option if you use the default. As of version 2.60,
spamd also supports communication over UNIX sockets. If you want to use
these, supply spamd_address with an absolute file name instead of a
address/port pair:

spamd_address = /var/run/spamd_socket

You can have multiple spamd servers to improve scalability. These can
reside on other hardware reachable over the network. To specify multiple
spamd servers, put multiple address/port pairs in the spamd_address
option, separated with colons:

Up to 32 spamd servers are supported. The servers are queried in a random
fashion. When a server fails to respond to the connection attempt, all other
servers are tried until one succeeds. If no server responds, the spam
condition defers.

Warning: It is not possible to use the UNIX socket connection method with
multiple spamd servers.

The spamd_address variable is expanded before use if it starts with
a dollar sign. In this case, the expansion may return a string that is
used as the list so that multiple spamd servers can be the result of an
expansion.

3. Calling SpamAssassin from an Exim ACL

Here is a simple example of the use of the spam condition in a DATA ACL:

deny message = This message was classified as SPAM
spam = joe

The right-hand side of the spam condition specifies a name. This is
relevant if you have set up multiple SpamAssassin profiles. If you do not want
to scan using a specific profile, but rather use the SpamAssassin system-wide
default profile, you can scan for an unknown name, or simply use “nobody”.
However, you must put something on the right-hand side.

The name allows you to use per-domain or per-user antispam profiles in
principle, but this is not straightforward in practice, because a message may
have multiple recipients, not necessarily all in the same domain. Because the
spam condition has to be called from a DATA ACL in order to be able to
read the contents of the message, the variables $local_part and $domain
are not set.

The right-hand side of the spam condition is expanded before being used, so
you can put lookups or conditions there. When the right-hand side evaluates to
“0” or “false”, no scanning is done and the condition fails immediately.

Scanning with SpamAssassin uses a lot of resources. If you scan every message,
large ones may cause significant performance degradation. As most spam messages
are quite small, it is recommended that you do not scan the big ones. For
example:

The spam condition returns true if the threshold specified in the user’s
SpamAssassin profile has been matched or exceeded. If you want to use the
spam condition for its side effects (see the variables below), you can make
it always return “true” by appending :true to the username.

When the spam condition is run, it sets up a number of expansion
variables. These variables are saved with the received message, thus they are
available for use at delivery time.

$spam_score

The spam score of the message, for example “3.4” or “30.5”. This is useful
for inclusion in log or reject messages.

$spam_score_int

The spam score of the message, multiplied by ten, as an integer value. For
example “34” or “305”. It may appear to disagree with $spam_score
because $spam_score is rounded and $spam_score_int is truncated.
The integer value is useful for numeric comparisons in conditions.

$spam_bar

A string consisting of a number of “+” or “-” characters, representing the
integer part of the spam score value. A spam score of 4.4 would have a
$spam_bar value of “++++”. This is useful for inclusion in warning
headers, since MUAs can match on such strings.

$spam_report

A multiline text table, containing the full SpamAssassin report for the
message. Useful for inclusion in headers or reject messages.

The spam condition caches its results unless expansion in
spamd_address was used. If you call it again with the same user name, it
does not scan again, but rather returns the same values as before.

The spam condition returns DEFER if there is any error while running
the message through SpamAssassin or if the expansion of spamd_address
failed. If you want to treat DEFER as FAIL (to pass on to the next ACL
statement block), append /defer_ok to the right-hand side of the
spam condition, like this:

4. Scanning MIME parts

The acl_smtp_mime global option specifies an ACL that is called once for
each MIME part of an SMTP message, including multipart types, in the sequence
of their position in the message. Similarly, the acl_not_smtp_mime option
specifies an ACL that is used for the MIME parts of non-SMTP messages. These
options may both refer to the same ACL if you want the same processing in both
cases.

These ACLs are called (possibly many times) just before the acl_smtp_data
ACL in the case of an SMTP message, or just before the acl_not_smtp ACL in
the case of a non-SMTP message. However, a MIME ACL is called only if the
message contains a Content-Type: header line. When a call to a MIME
ACL does not yield “accept”, ACL processing is aborted and the appropriate
result code is sent to the client. In the case of an SMTP message, the
acl_smtp_data ACL is not called when this happens.

You cannot use the malware or spam conditions in a MIME ACL; these can
only be used in the DATA or non-SMTP ACLs. However, you can use the regex
condition to match against the raw MIME part. You can also use the
mime_regex condition to match against the decoded MIME part (see section
43.5).

At the start of a MIME ACL, a number of variables are set from the header
information for the relevant MIME part. These are described below. The contents
of the MIME part are not by default decoded into a disk file except for MIME
parts whose content-type is “message/rfc822”. If you want to decode a MIME
part into a disk file, you can use the decode condition. The general
syntax is:

decode = [/<path>/]<filename>

The right hand side is expanded before use. After expansion,
the value can be:

“0” or “false”, in which case no decoding is done.

The string “default”. In that case, the file is put in the temporary
“default” directory <spool_directory>/scan/<message_id>/ with
a sequential file name consisting of the message id and a sequence number. The
full path and name is available in $mime_decoded_filename after decoding.

A full path name starting with a slash. If the full name is an existing
directory, it is used as a replacement for the default directory. The filename
is then sequentially assigned. If the path does not exist, it is used as
the full path and file name.

If the string does not start with a slash, it is used as the
filename, and the default path is then used.

The decode condition normally succeeds. It is only false for syntax
errors or unusual circumstances such as memory shortages. You can easily decode
a file with its original, proposed filename using

decode = $mime_filename

However, you should keep in mind that $mime_filename might contain
anything. If you place files outside of the default path, they are not
automatically unlinked.

For RFC822 attachments (these are messages attached to messages, with a
content-type of “message/rfc822”), the ACL is called again in the same manner
as for the primary message, only that the $mime_is_rfc822 expansion
variable is set (see below). Attached messages are always decoded to disk
before being checked, and the files are unlinked once the check is done.

The MIME ACL supports the regex and mime_regex conditions. These can be
used to match regular expressions against raw and decoded MIME parts,
respectively. They are described in section 43.5.

The following list describes all expansion variables that are
available in the MIME ACL:

$mime_boundary

If the current part is a multipart (see $mime_is_multipart) below, it should
have a boundary string, which is stored in this variable. If the current part
has no boundary parameter in the Content-Type: header, this variable
contains the empty string.

$mime_charset

This variable contains the character set identifier, if one was found in the
Content-Type: header. Examples for charset identifiers are:

us-ascii
gb2312 (Chinese)
iso-8859-1

Please note that this value is not normalized, so you should do matches
case-insensitively.

$mime_content_description

This variable contains the normalized content of the Content-Description:
header. It can contain a human-readable description of the parts content. Some
implementations repeat the filename for attachments here, but they are usually
only used for display purposes.

$mime_content_disposition

This variable contains the normalized content of the Content-Disposition:
header. You can expect strings like “attachment” or “inline” here.

$mime_content_id

This variable contains the normalized content of the Content-ID: header.
This is a unique ID that can be used to reference a part from another part.

$mime_content_size

This variable is set only after the decode modifier (see above) has been
successfully run. It contains the size of the decoded part in kilobytes. The
size is always rounded up to full kilobytes, so only a completely empty part
has a $mime_content_size of zero.

$mime_content_transfer_encoding

This variable contains the normalized content of the
Content-transfer-encoding: header. This is a symbolic name for an encoding
type. Typical values are “base64” and “quoted-printable”.

$mime_content_type

If the MIME part has a Content-Type: header, this variable contains its
value, lowercased, and without any options (like “name” or “charset”). Here
are some examples of popular MIME types, as they may appear in this variable:

text/plain
text/html
application/octet-stream
image/jpeg
audio/midi

If the MIME part has no Content-Type: header, this variable contains the
empty string.

$mime_decoded_filename

This variable is set only after the decode modifier (see above) has been
successfully run. It contains the full path and file name of the file
containing the decoded data.

$mime_filename

This is perhaps the most important of the MIME variables. It contains a
proposed filename for an attachment, if one was found in either the
Content-Type: or Content-Disposition: headers. The filename will be
RFC2047 decoded, but no additional sanity checks are done. If no filename was
found, this variable contains the empty string.

$mime_is_coverletter

This variable attempts to differentiate the “cover letter” of an e-mail from
attached data. It can be used to clamp down on flashy or unnecessarily encoded
content in the cover letter, while not restricting attachments at all.

The variable contains 1 (true) for a MIME part believed to be part of the
cover letter, and 0 (false) for an attachment. At present, the algorithm is as
follows:

The outermost MIME part of a message is always a cover letter.

If a multipart/alternative or multipart/related MIME part is a cover letter,
so are all MIME subparts within that multipart.

If any other multipart is a cover letter, the first subpart is a cover letter,
and the rest are attachments.

All parts contained within an attachment multipart are attachments.

As an example, the following will ban “HTML mail” (including that sent with
alternative plain text), while allowing HTML files to be attached. HTML
coverletter mail attached to non-HMTL coverletter mail will also be allowed:

This variable has the value 1 (true) when the current part has the main type
“multipart”, for example “multipart/alternative” or “multipart/mixed”.
Since multipart entities only serve as containers for other parts, you may not
want to carry out specific actions on them.

$mime_is_rfc822

This variable has the value 1 (true) if the current part is not a part of the
checked message itself, but part of an attached message. Attached message
decoding is fully recursive.

$mime_part_count

This variable is a counter that is raised for each processed MIME part. It
starts at zero for the very first part (which is usually a multipart). The
counter is per-message, so it is reset when processing RFC822 attachments (see
$mime_is_rfc822). The counter stays set after acl_smtp_mime is
complete, so you can use it in the DATA ACL to determine the number of MIME
parts of a message. For non-MIME messages, this variable contains the value -1.

5. Scanning with regular expressions

You can specify your own custom regular expression matches on the full body of
the message, or on individual MIME parts.

The regex condition takes one or more regular expressions as arguments and
matches them against the full message (when called in the DATA ACL) or a raw
MIME part (when called in the MIME ACL). The regex condition matches
linewise, with a maximum line length of 32K characters. That means you cannot
have multiline matches with the regex condition.

The mime_regex condition can be called only in the MIME ACL. It matches up
to 32K of decoded content (the whole content at once, not linewise). If the
part has not been decoded with the decode modifier earlier in the ACL, it
is decoded automatically when mime_regex is executed (using default path
and filename values). If the decoded data is larger than 32K, only the first
32K characters are checked.

The regular expressions are passed as a colon-separated list. To include a
literal colon, you must double it. Since the whole right-hand side string is
expanded before being used, you must also escape dollar signs and backslashes
with more backslashes, or use the \N facility to disable expansion.
Here is a simple example that contains two regular expressions:

The conditions returns true if any one of the regular expressions matches. The
$regex_match_string expansion variable is then set up and contains the
matching regular expression.

Warning: With large messages, these conditions can be fairly
CPU-intensive.

6. The demime condition

The demime ACL condition provides MIME unpacking, sanity checking and file
extension blocking. It is usable only in the DATA and non-SMTP ACLs. The
demime condition uses a simpler interface to MIME decoding than the MIME
ACL functionality, but provides no additional facilities. Please note that this
condition is deprecated and kept only for backward compatibility. You must set
the WITH_OLD_DEMIME option in Local/Makefile at build time to be able to
use the demime condition.

The demime condition unpacks MIME containers in the message. It detects
errors in MIME containers and can match file extensions found in the message
against a list. Using this facility produces files containing the unpacked MIME
parts of the message in the temporary scan directory. If you do antivirus
scanning, it is recommended that you use the demime condition before the
antivirus (malware) condition.

On the right-hand side of the demime condition you can pass a
colon-separated list of file extensions that it should match against. For
example:

If one of the file extensions is found, the condition is true, otherwise it is
false. If there is a temporary error while demimeing (for example, “disk
full”), the condition defers, and the message is temporarily rejected (unless
the condition is on a warn verb).

The right-hand side is expanded before being treated as a list, so you can have
conditions and lookups there. If it expands to an empty string, “false”, or
zero (“0”), no demimeing is done and the condition is false.

The demime condition set the following variables:

$demime_errorlevel

When an error is detected in a MIME container, this variable contains the
severity of the error, as an integer number. The higher the value, the more
severe the error (the current maximum value is 3). If this variable is unset or
zero, no error occurred.

$demime_reason

When $demime_errorlevel is greater than zero, this variable contains a
human-readable text string describing the MIME error that occurred.

$found_extension

When the demime condition is true, this variable contains the file
extension it found.

Both $demime_errorlevel and $demime_reason are set by the first call of
the demime condition, and are not changed on subsequent calls.

If you do not want to check for file extensions, but rather use the demime
condition for unpacking or error checking purposes, pass “*” as the
right-hand side value. Here is a more elaborate example of how to use this
facility: